1. Field of the Invention
The present invention relates to a method for driving a semiconductor device including a photosensor, particularly to a method for driving the semiconductor device using the time of flight (TOF) method.
2. Description of the Related Art
There has been increasing interest in three-dimensional distance-measuring sensors which can detect the distance from a light source to an object with a detection signal dependent on the time difference between the arrival times of light. Three-dimensional distance-measuring sensors are expected to have a wide range of applications such as virtual keyboards, devices for input of information obtained by gesture recognition and the like, visual sensors in various robots, security systems, sensors for smart air bags, and sensors for cars.
Moving body detecting sensors are used for crime prevention, such as intruder detection, and detection of unusual events, such as falls in a restroom or bathroom, in consideration of, for example, the advance of the aging society and consumer needs that require safer and more comfortable living environments. In order to detect the conditions of a moving person or object more accurately, a technique for obtaining accurate information on a moving body (e.g., the presence, position, motion, and size of an object) has been developed.
The TOF method is a method in which time difference between the time at which light emitted from a light source (a distance-measuring sensor), called “irradiation light”, arrives at an object and the time at which light reflected off the object, called “reflected light”, arrives at the distance-measuring sensor is detected, and the distance from the light source to the object is obtained by calculation. The distance from the light source to the object (distance x) can be expressed by Equation 1, where c is the speed of light (3×108 m/s), and Δt is the time difference (see FIG. 5).
                    x        =                              (                          c              ×              Δ              ⁢                                                          ⁢              t                        )                    2                                    [                  Equation          ⁢                                          ⁢          1                ]            
As an example of a three-dimensional distance-measuring sensor, Non Patent Document 1 discloses a three-dimensional distance-measuring device that performs three-dimensional distance measurement by performing, with the TOF method, three-dimensional imaging in which a period for detecting reflected infrared light is divided into two periods to obtain different detection signals.
According to Non Patent Document 2, two-dimensional imaging that provides two-dimensional information (e.g. the intensity of light reflected off an object and the color of the object) and three-dimensional imaging that provides three-dimensional information (the distance from a light source to an object) are alternately performed every frame period using the structure of a photosensor according to Non Patent Document 1. The three-dimensional imaging is performed with the TOF method by emitting infrared light more than once.